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  <div class="section" id="module-copy">
<h1>9.17. <tt class="xref docutils literal"><span class="pre">copy</span></tt> &#8212; Shallow and deep copy operations<a class="headerlink" href="#module-copy" title="Permalink to this headline">¶</a></h1>
<p id="index-242">This module provides generic (shallow and deep) copying operations.</p>
<p>Interface summary:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">copy</span>

<span class="n">x</span> <span class="o">=</span> <span class="n">copy</span><span class="o">.</span><span class="n">copy</span><span class="p">(</span><span class="n">y</span><span class="p">)</span>        <span class="c"># make a shallow copy of y</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">copy</span><span class="o">.</span><span class="n">deepcopy</span><span class="p">(</span><span class="n">y</span><span class="p">)</span>    <span class="c"># make a deep copy of y</span>
</pre></div>
</div>
<p>For module specific errors, <tt class="xref docutils literal"><span class="pre">copy.error</span></tt> is raised.</p>
<p>The difference between shallow and deep copying is only relevant for compound
objects (objects that contain other objects, like lists or class instances):</p>
<ul class="simple">
<li>A <em>shallow copy</em> constructs a new compound object and then (to the extent
possible) inserts <em>references</em> into it to the objects found in the original.</li>
<li>A <em>deep copy</em> constructs a new compound object and then, recursively, inserts
<em>copies</em> into it of the objects found in the original.</li>
</ul>
<p>Two problems often exist with deep copy operations that don&#8217;t exist with shallow
copy operations:</p>
<ul class="simple">
<li>Recursive objects (compound objects that, directly or indirectly, contain a
reference to themselves) may cause a recursive loop.</li>
<li>Because deep copy copies <em>everything</em> it may copy too much, e.g.,
administrative data structures that should be shared even between copies.</li>
</ul>
<p>The <tt class="xref docutils literal"><span class="pre">deepcopy()</span></tt> function avoids these problems by:</p>
<ul class="simple">
<li>keeping a &#8220;memo&#8221; dictionary of objects already copied during the current
copying pass; and</li>
<li>letting user-defined classes override the copying operation or the set of
components copied.</li>
</ul>
<p>This module does not copy types like module, method, stack trace, stack frame,
file, socket, window, array, or any similar types.  It does &#8220;copy&#8221; functions and
classes (shallow and deeply), by returning the original object unchanged; this
is compatible with the way these are treated by the <a title="Convert Python objects to streams of bytes and back." class="reference external" href="pickle.html#module-pickle"><tt class="xref docutils literal"><span class="pre">pickle</span></tt></a> module.</p>
<p>Shallow copies of dictionaries can be made using <a title="dict.copy" class="reference external" href="stdtypes.html#dict.copy"><tt class="xref docutils literal"><span class="pre">dict.copy()</span></tt></a>, and
of lists by assigning a slice of the entire list, for example,
<tt class="docutils literal"><span class="pre">copied_list</span> <span class="pre">=</span> <span class="pre">original_list[:]</span></tt>.</p>
<p>
<span class="versionmodified">Changed in version 2.5: </span>Added copying functions.</p>
<p id="index-243">Classes can use the same interfaces to control copying that they use to control
pickling.  See the description of module <a title="Convert Python objects to streams of bytes and back." class="reference external" href="pickle.html#module-pickle"><tt class="xref docutils literal"><span class="pre">pickle</span></tt></a> for information on these
methods.  The <tt class="xref docutils literal"><span class="pre">copy</span></tt> module does not use the <a title="Register pickle support functions." class="reference external" href="copy_reg.html#module-copy_reg"><tt class="xref docutils literal"><span class="pre">copy_reg</span></tt></a> registration
module.</p>
<p id="index-244">In order for a class to define its own copy implementation, it can define
special methods <tt class="xref docutils literal"><span class="pre">__copy__()</span></tt> and <tt class="xref docutils literal"><span class="pre">__deepcopy__()</span></tt>.  The former is called
to implement the shallow copy operation; no additional arguments are passed.
The latter is called to implement the deep copy operation; it is passed one
argument, the memo dictionary.  If the <tt class="xref docutils literal"><span class="pre">__deepcopy__()</span></tt> implementation needs
to make a deep copy of a component, it should call the <tt class="xref docutils literal"><span class="pre">deepcopy()</span></tt> function
with the component as first argument and the memo dictionary as second argument.</p>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt>Module <a title="Convert Python objects to streams of bytes and back." class="reference external" href="pickle.html#module-pickle"><tt class="xref docutils literal"><span class="pre">pickle</span></tt></a></dt>
<dd>Discussion of the special methods used to support object state retrieval and
restoration.</dd>
</dl>
</div>
</div>


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